During the past decade, there has been a birth of interest in a class of devices commonly referred to as monolithically integrated modulators with distributed feedback (DFB) lasers or distributed Bragg reflector (DBR) lasers. This spark of interest has been attributed to the fact that these structures evidence low wavelength chirp characteristics which are of particular interest for use in high speed lightwave communication systems.
Heretofore, in the fabrication of such devices, it has been the practice of workers in the art to integrate modulators with active elements such as lasers or amplifiers by the use of either butt joint couplings or selective area epitaxy. The butt joint coupling technique is limited in that epitaxial growth required for fabrication of the device of interest must be effected in more than one procedural steps. Unfortunately, this type of processing ofttimes results in the introduction of rough interfaces on the surface of the structure, particularly at those sites where it is desirable to effect joinder of the laser with the modulator. This typically leads to the deleterious scattering of light in the finished device.
Efforts to alleviate this difficulty focused upon the use of a technique commonly referred to as selective area epitaxy. In this process, a dielectric layer in the form of a thin film is deposited on the surface of the substrate prior to effecting growth of an epitaxial layer. This dielectric layer, which typically comprises silicon dioxide, also tends to introduce roughness upon the surface of the substrate and, more importantly, leads to the formation of a polycrystalline film grown on the surface of the dielectric which may interfere with further processing. Accordingly, workers in the art have continued their search for new techniques designed to eliminate the prior art deficiencies.